This is a continuation-in-part of International Application PCT/JP01/03172 with an international filing date of Apr. 12, 2001, published in Japanese on Nov. 1, 2001 under PCT Article 21(2).
The present invention relates to a process for producing organic halogen compound-decomposing catalysts to be used in treating waste gases mainly from incinerators, and a process for producing organic halogen compound-decomposing filters.
There have been improvements in burning conditions and advancements in waste gas treatment to reduce organic halogen compounds generated from city garbage incinerators. In general, high temperatures of at least 1000xc2x0 C. are required to be maintained to completely oxidize and decompose halogenated organic substances such as organic halogen compounds in burning processes, but burning temperatures are usually set at 800 to 900xc2x0 C. to prevent damage to furnace walls. It is not easy to completely oxidize and decompose the organic halogen compounds represented by organic chlorine compounds merely through improving burning conditions.
In a stoker furnace, where low-temperature areas or an incomplete burning area tend to be locally formed, a re-burning section needs to be provided to completely oxidize and decompose the organic halogen compounds. However, this poses problems in that the total amount of waste gases increases, and running with re-burning fuel causes costs to rise.
It is known that among the organic halogen compounds, dioxins and brominated dioxins are produced not only in the burning process, but also such are synthesized again from non-pyrolysed and unburnt residual portions and benzene ring-possessing precursor materials over a wide temperature zone of around 300 to 500xc2x0 C. Therefore, even when attempts are made to reduce the dioxins and the brominated dioxins in the burning process, these are synthesized again in a succeeding stage. Consequently, the dioxins and the brominated dioxins in the waste gases are sometimes not reduced.
For the above reasons, the organic halogen compounds in the waste gases, including the dioxins and the brominated dioxins, need to be removed through waste gas treatment so that they may be kept stably at low concentrations.
Adsorption methods include adsorption with activated carbon and decomposition with a catalyst for the removal of the organic halogen compounds in the waste gases, including the dioxins and the brominated dioxins through the waste gas treatment. Although this adsorption method is the mainstream in the existing treatment of the organic halogen compounds, it is costly, and poses a problem in that activated carbon, once having adsorbed the dioxins, becomes a secondary waste, which requires a post treatment thereof. On the other hand, although the catalytically decomposing method has merit in that no secondary waste is produced, it has problems in that the dioxins are synthesized again through secondary reactions with the catalyst and the catalyst is degraded.
Although denitriding catalysts of a V2O5-TiO2 system and a V2O5-WO3TiO2 system have been formerly used as organic chloride compound-decomposing catalysts, it has been confirmed that dioxins are produced, contrary to expectations, particularly if the concentration of the dioxins at the inlet for the catalyst is low. In recent years, in many areas where the concentration of dioxins in the waste gas is suppressed to not more than {fraction (1/10)} of the governmental standard value set by residential agreements or the like, it was clarified that using such conventional organic halogen compound-decomposing catalysts might afford adverse effects in view of the above considerations. Thus, the re-synthesis of the dioxins with the catalyst becomes a problem. Further, since such a conventional catalyst needs to be used at high temperatures of 200 to 500xc2x0 C., the problem that the waste gases need to be heated again comes out.
Moreover, investigations and research have recently been more actively made on the level of influence upon human health caused by dioxins and brominated dioxins produced during the burning processes of the wastes and the treating processes of the waste gases, as well as on processes through which they are produced.
Having thus examined the re-synthesis phenomena of dioxins and brominated dioxins with the catalysts, the inventors confirmed that V2O5 causes the above, and that if Pt and/or Pd is used, catalysts which decompose organic halogen compounds including the dioxins and the brominated dioxins can be obtained, free from the re-synthesis problem. A method which is ordinarily considered to produce titanium in which Pt and/or Pd is dispersed is a method of coating a powder of titanium oxide with an aqueous solution of a Pt salt and/or a Pd salt or impregnating a sintered body thereof with this solution, heating the resultant, decomposing the salt(s) and thereby precipitating Pt and/or Pd thereon.
In the organic halogen compound-decomposing catalyst obtained by this method, however, the Pt and/or Pd particles have large diameters, and precipitate on the surface of titanium oxide uniformly. Thus, there is a problem that a catalytic effect is not effectively exhibited if the particle diameters of titanium oxide are small. In addition, the titniumoxide particles are sintered, and titanium oxide and noble metal particles are simultaneously grain-grown (sintered), if the particles are heated at high temperatures. It causes degradation in the catalytic activity. Summary of the Invention The present invention has been accomplished to provide a process for producing an organic halogen compound-decomposing catalyst which can solve the above-mentioned problems of the prior art, increase active points by uniformly dispersing Pt and/or Pd fine particles in titanium particles, and assuredly oxidize and decompose the organic halogen compounds including the dioxins and the brominated dioxins, without fear of re-synthesizing dioxins and brominated dioxins, even if the catalytic reaction temperature is lowered to 200xc2x0 C. or less.
It is another object of the present invention to provide a process for producing an organic halogen compound-decomposing filter by using the above organic halogen compound-decomposing catalyst.
The process for producing the organic halogen compound-decomposing catalyst according to the present invention, which process has been made to solve the above problems, comprises the steps of synthesizing an organic metal precursor containing Pt and/or Pd and Ti in molecules thereof by reacting a Pt salt and/or a Pd salt, an organic Ti compound and an organic binder in a solvent, heating the organic metal precursor, and thereby obtaining the organic halogen compound-decomposing catalyst in which fine particles of Pt and/or Pd are dispersed in TiO2.
The process for producing an organic halogen compound-decomposing filter according to the present invention, comprises the steps of synthesizing an organic metal precursor containing Pt and/or Pd and Ti in molecules thereof by reacting a Pt salt and/or a Pd salt, an organic Ti compound, and an organic binder in a solvent, coating the organic metal precursor on a surface of a clean side of a ceramic filter or coating the precursor on wall surfaces of inner pores of the filter by impregnation, heating the resulting filter, and thereby carrying on the ceramic filter the organic halogen compound-decomposing catalyst in which fine particles of Pt and/or Pd are dispersed in TiO2.
In the organic halogen compound-decomposing catalyst obtained by the process of the present invention, the Pt and/or Pd fine particles are dispersed in the TiO2 by heating the organic metal precursor containing Pt and/or Pd and Ti in its molecules. Therefore, as compared with conventional catalysts obtained by coating or impregnation, the catalyst of the present invention has largely increased active points, and can be highly activated. Thus, the amount of expensive Pt or Pd can be reduced to cut the cost. The organic metal precursor is ordinarily heated at a heating temperature of 450to 500xc2x0 C. in an air atmosphere under atmospheric pressure. The heating time is set at such a sufficient length in connection with the heating temperature, etc. that the organic binder is completely burnt out and that Pt or Pd is uniformly dispersed in TiO2. Further, since the organic halogen compound can be decomposed even under a low temperature condition of less than 200xc2x0 C., the waste gases need not be heated again, and the organic halogen compound is not synthesized again. In the present invention, xe2x80x9cPt or Pdxe2x80x9d means xe2x80x9cPt and/or Pd.xe2x80x9d
In the organic halogen compound-decomposing filter obtained by the process of the present invention, the organic halogen compound-decomposing catalyst having such functions is carried on the surface of the clean side of the ceramic filter or on the inner filtering wall surfaces of the filter. Thus, the filter has a merit that dust is removed with simultaneous decomposition of the organic halogen compounds.